(a) The Invention
The invention relates to a novel tubular article and to a device comprising one or more sensors surrounded by said tubular article, said device being useful for towing behind a vessel at sea as a towed-array sensor system.
(b) Background Invention
It is desirable to enclose articles such as electronic devices, transmitters, receivers or sensing instruments, (e.g. hydrophones used in sonar) in a tube which will protect them from the effects of water. Such tubing, which may be 100 meters or more in length, can then be used to make readings when towed by a vessel in the open sea. Typically, hydrophones are packed into a semi-rigid tube typically 1 to about 4 inches in diameter. The tubing is loaded with an acoustically transparent material such as oil through which sound may propagate and which may add buoyancy. The hydrophones are fixed in place by tying each of the string of hydrophones to the tube and sealing the tube.
This entire assemblage is typically known as a "towed-array" and is used as the sensing portion of a sonar system for both civilian and military applications.
It will be readily apparent that a high sensitivity of detection is desirable and that since this sensitivity is partly determined by the signal-to-noise ratio reaching the hydrophones, it is advantageous to reduce the acoustic noise generated within or by the array. As the array is pulled through the water, noise is generated from three separate mechanical excitations. The first is towline excitation originating either at the vessel or as towline strumming due to vortex shedding. The second is a tail end effect; array tension is low and the array may be susceptible to "snaking" instability. The third, and most important, noise source is the turbulent boundary layer (TBL) which develops as the array is drawn through water. The way in which the TBL creates noise is by generating bulge waves and/or extensional waves (either resonant or non-resonant) in the hosewall. The entire success of towed arrays depends on the isolation of the hydrophones from these surface pressures (whose spectral level increases as the fourth power of speed).
It is desirable, then, to use a tubing which provides for maximum transmission of the acoustic signals that the sensors are attempting to detect but which minimizes the noise created by the array and minimizes transmission of the noise. Two major properties of the material selected for tubing affect its acoustic properties. One property of material that significantly reduces noise is its ability to exhibit high mechanical damping. A second major property of the tubing is that it should not be too flexible, (i.e. should not have a low modulus) as this will more readily encourage the generation of bulge waves due to the TBL. This problem is further exacerbated by the fact that towed array tubes are limited in their wall thickness due to the size of the hydrophones, the degree of difficulty in towing large diameter tubing and the need to be radially compliant to permit it to be easily coiled for stronger handling purposes. Stiffness, therefore, due to wall thickness can not be changed to a great degree. Secondary considerations include the density of the material, abrasion resistance, resistance to the fill fluids and water, low temperature flexibility, creep resistance, the ability to take a good surface finish, and the ability to manufacture in continuous lengths.
Compositions that have been used for jacketing sonar detection arrays include plasticized polyvinyl chloride (PVC) and various rubbers, e.g. butyl rubber and nitrile rubber. However, while the elastomers have high mechanical damping, the elastomers suffer from disadvantages such as poor processing characteristics (especially for long lengths), poor abrasion resistance, insufficient stiffness, excessive weight and insufficient solvent resistance. Plasticized PVC can be easily processed, but is too stiff at low temperatures and the plasticizer is leached out by the acoustic fluids. Further the elastomers tend to be of relatively low modulus and thus tend to flex more often resulting in the creation of more noise. One method to achieve a balance between modulus and damping effects has been to blend an acoustic damping material with a material of higher modulus. However, these materials tend to be difficult and expensive to process into continuous tubing lengths.
In U.S. Pat. No. 4,410,012, a line array hose is described comprising a tube of soft flexible, thermoplastic material having high damping characteristics and a plurality of raised longitudinal ribs. The construction is described as having good self-noise damping properties and improved resistance to mechanical abuse. However, the raised longitudinal ribs cause an increased turbulence and noise at the TBL. Additionally, the hose is made of PVC and therefore has a high specific gravity.
In commonly assigned U.K. Application GB No. 2,123,011A, incorporated herein by reference, compositions comprising a segmented urethane block polymer and an elastomer having a glass transition temperature between -40.degree. C. and +15.degree. C. are described. These compositions maintain relatively high damping values of the elastomer while increasing modulus values.
It has been surprisingly found that a tubular article of the invention has good damping characteristics while maintaining a high modulus and is easy to process at relatively low cost.